This invention relates to the field of martensitic memory alloys and more particularly to a method for annealing martensitic nickel/titanium alloys to substantially improve their tensile strength and to improve the ability of the alloys to retain their original properties during use.
Alloys of nickel and titanium in which the two elements are present in roughly the same molar proportions have been demonstrated to have martensitic memory properties rendering them highly useful in control devices and other services in which temperature actuation is desirable. When placed under stress, an alloy roughly corresponding to the formula NiTi undergoes a martensitic phase transformation in a relatively narrow temperature range with a resultant change in dimension. This dimensional change is negative with respect to temperature. Thus, if an NiTi wire is under tension and is cooled from a temperature above the martensitic transformation range, it will elongate when a critical temperature range is reached. Conversely, when the wire is heated from a temperature below the martensitic range, it will shorten in a temperature range in which the phase transformation is reversed. In such thermal cycling of the wire there is a hysteresis effect in that the major share of the reverse transformation takes place in a temperature range somewhat higher than the temperatures at which the major share of elongation takes place. This phenomenon is illustrated in FIG. 1. The phase transformation associated with elongation is accompanied by the release of heat energy and the reverse transformation is accompanied by an absorption of heat.
Because of their unique property of elongating and reversibly foreshortening over a relatively narrow temperature range, martensitic memory alloys, such as nickel/titanium, have found application as thermostatic elements in control devices and as means for the conversion of heat energy to mechanical energy in devices for performing work. Where the alloy is in the form of a thin wire, for example, it may be very rapidly heated or cooled to cause sharp changes in dimension. The practical utility of such a device is enhanced by the extent of this change in dimension. The martensitic elongation activity of these alloys, defined as the ratio of change in length to length (.DELTA. L/L) expressed as a percentage, ranges as high as 2-6%.
The usefulness of nickel/titanium alloys has been somewhat limited, however, by certain disadvantageous properties. It has been observed, for example, that when a nickel/titanium element is carried through a series of temperature cycles about its martensitic transformation range, it does not fully return to its original dimension but instead progressively elongates or relaxes with each cycle. This phenomenon, which is hereinafter referred to as cyclic creep and which is illustrated in FIG. 2, is a serious obstacle to the practical utility of the nickel/titanium alloy.
A further problem associated with nickel/titanium alloys results from the fact that their martensitic transition temperature is typically near room temperature. As a consequence, the alloy may tend to undergo phase transformations and resultant elongations and foreshortenings due to ambient variations alone. This characteristic presents obvious difficulties in the use of nickel/titanium alloys in control-actuating devices responsive to variables other than ambient temperature.
The transformation temperature of nickel-titanium alloys can be altered by the amount of stress under which the alloys are placed. Thus, for example, if a nickel/titanium wire is placed under relatively high tension, the temperatures at which the phase transformation takes place may be increased by as much as 70.degree. C. However, the feasibility of realizing such substantial increases in the operating temperature of a nickel/titanium device may be seriously limited by the tensile strength of the alloy itself. Even when the stress is not sufficient to cause the alloy to yield or fail, moreover, NiTi alloys are subject to creep at elevated temperature. This creep, of course, adversely effects dimensional stability independently of the cyclic creep or progressive elongation due to thermal cycling.
A practical need has thus existed for methods to stabilize martensitic nickel/titanium alloys against progressive elongation or relaxation, and to increase their tensile strength and cyclic creep resistance so that they may be placed under high stress and used in circumstances where they respond at significantly elevated temperatures. Efforts have been made in the art to meet each of these objectives. Thus, for example, Willson et al. U.S. Pat. No. 3,652,969 describes a method in which the nickel/titanium alloy is repeatedly cycled through its critical temperature range while it is maintained under stress substantially greater than the stress to be applied in an anticipated practical application. Although thus useful for improving the repeatability of a nickel/titanium alloy device and preventing relaxation during use, the method of Willson et al. is not directed towards any improvement in the tensile strength.
Wang U.S. Pat. No. 3,594,239 describes a process which is also directed to minimizing the relaxation of martensitic memory alloys during use. This process involves annealing the alloy at 650.degree.-700.degree. C. and slowly cooling it to a temperature below that at which it undergoes thermal cycling. Wang also contemplates a further step of thermal cycling between the upper critical temperature limit and the lower critical temperature limit as a means of increasing the maximum resistivity value in the martensitic range, but does not suggest the application of stress during this step as taught by Willson et al. Like Willson et al., Wang is primarily concerned with avoiding the relaxation resulting from thermal cycling and, although he does recognize the value of applying stress to increase the temperature at which martensitic transformation takes place, Wang is not concerned with increasing the tensile strength to maximize the stress that can be applied.
Others, as described for example in Rozner and Buehler U.S. Pat. No. 3,351,463, have been concerned with improving the mechanical strength of nickel/titanium alloys but not with the problem of progressive elongation due to thermal cycling. The progress of Rozner and Buehler involves working the alloy below its critical temperature, typically by the methods used in shaping and fabrication. Working is carried out after annealing in this process with a consequent materially adverse effect on the elongation activity of the alloy.